Higher operating temperatures of gas turbine engines are continually being sought in order to increase the efficiency of the engines. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel, cobalt and iron based superalloys. These superalloys can be designed to withstand temperatures in the range of about 1000 to about 1100° C. or higher. Nonetheless, when used to form components of the turbine, such as combustor liners, augmentor hardware, shrouds and high and low-pressure nozzles and blades, the superalloys alone could be susceptible to damage by oxidation and hot corrosion attack. Accordingly, these components are typically protected by an environmental and/or a thermal barrier coating (TBC). In general, TBCs can be used in conjunction with the superalloys in order to reduce the cooling air requirements associated with a given turbine. Ceramic materials, such as yttrium-stabilized zirconia (YSZ), are widely used as a TBC or topcoat of TBC systems. These materials are employed because, for example, they can be readily deposited by plasma-spraying and physical vapor deposition (PVD) techniques, and they also generally exhibit desirable thermal characteristics. In general, these TBCs can be utilized in conjunction with the superalloys in order to reduce the cooling air requirements associated with a given turbine.
In order to be effective, TBCs need to possess low thermal conductivity, strongly adhere to the component and remain adhered through many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between the ceramic materials and the superalloy substrates that they protect. To promote adhesion and extend the service life of a TBC, an oxidation-resistant bond coating typically takes the form of a diffusion aluminide coating or an overlay coating, such as MCrAlX where M is iron, cobalt and/or nickel and X is yttrium or another rare earth element. During the deposition of a ceramic TBC and subsequent exposures to high temperatures, such as during engine operation, these bond coats form a tightly adherent alumina (Al2O3) layer or scale that adheres the TBC to the bond coat.
The service life of a TBC is typically limited by a spallation event brought on by, for example, thermal fatigue. Accordingly, a significant challenge has been to obtain a more adherent ceramic layer that is less susceptible to spalling when subjected to thermal cycling. Though significant advances have been made, there is the inevitable requirement to repair components whose thermal barrier coatings have spalled. Though spallation typically occurs in localized regions or patches, a conventional repair method has been to completely remove the TBC after removing the affected component from the turbine or other area, restore or repair the bond coat as necessary and recoat the engine component. Techniques for removing TBCs include grit blasting or chemically stripping with an alkaline solution at high temperatures and pressures. However, grit blasting is a slow, labor-intensive process and can erode the surface beneath the coating. The use of an alkaline solution to remove a TBC also is less than ideal because the process typically requires the use of an autoclave operating at high temperatures and pressures. Consequently, some conventional repair methods are labor intensive and expensive, and can be difficult to perform on components with complex geometries, such as airfoils and shrouds. As an alternative, U.S. Pat. No. 5,723,078 to Nagaraj et al. teach selectively repairing a spalled region of a TBC by texturing the exposed surface of the bond coat, and then depositing a ceramic material on the textured surface by plasma spraying. While avoiding the necessity to strip the entire TBC from a component, the repair method taught by Nagaraj et al. requires removal of the component in order to deposit the ceramic material.
In the case of large power generation turbines, completely halting power generation for an extended period of time in order to remove components whose TBCs have suffered only localized spallation is not economically desirable.
U.S. Pat. No. 7,476,703 to Ruud et al. discloses an in-situ method and composition for repairing a thermal barrier coating, which is based on a silicone resin system. While this in-situ method alleviates the disassembly, masking and over-spraying problems associated with some conventional TBC repair methods, it is not an ideal repair for large area defects (i.e., defects that are greater than 1 square inch in size). U.S. Pat. No. 6,413,578 to Stowell et al. discloses an in-situ method for repairing thermal barrier coating with a ceramic paste. However, this method uses a repair composition that contains ethyl alcohol. As a result, flammable ethyl alcohol fumes are released when the repair composition is used, which creates environmental health and safety risks.
A commercially available repair composition, AIM-MRO SR Resin Patch, may be used for TBC repair. However, this repair composition is silicate based and for this reason does not offer the desired performance of thermal barrier coating. Additionally, the commercial repair composition cannot be used to repair large area defects, such as when the damaged area is greater than 1 square inch in size.
Accordingly, despite the above advances, it would be desirable if a repair method and a repair composition were available that could be performed on damaged regions of various sizes, including large damaged regions (i.e., damaged regions that are greater than 1 square inch in size), without necessitating that the component be removed from the turbine, so that downtime and scrappage are minimized. Such damaged regions may be created by localized spallation, damage caused by tool hits, and/or chipping. Furthermore, it would be desirable to have a repair composition that uses water as a liquid carrier, thus avoiding environmental health and safety risks associated with repair compositions that use organic solvents, such as ethyl alcohol.